The invention relates to a surface acoustic wave sensor comprising at least two oscillating passive components having oscillator circuits as measuring oscillators disposed in a housing through which a medium to be tested is conducted.
Such a sensor is known for example from DE 44 17 170.
Modified acoustic surface wave--or Surface Acoustic Wave (SAW) building components can be employed for the chemical sensing of gases or liquids by applying a respective chemically reactive coating to the components. With the ab- and adsorption of the analyte, the mass of the coating as well as the elastic parameters thereof change, whereby the sound propagation speed of the surface wave changes. In order to measure the change of the sound propagation speed of a surface wave in a simple manner, it is common practice to include in an oscillator circuit a coated SAW component, which is used as a frequency determination element.
The sound propagation speed change results in a proportional change of the oscillator frequency, which can be determined with a good approximation and can be measured with a high resolution of typically 10.sup.-6. With an appropriate selection of sorption layers, almost any number of gaseous analytes can be examined with this technique. Of greatest interest are those materials for which qualitative and quantitative determinations are difficult to perform with other chemical microsensors. Those are organic solvents such as hydrocarbons (hexane octane, decane, various fuels), alcohols (methanol, ethanol, halogenized hydrocarbons (CKW's, FCKW's), and aromatics (benzene, toluene).
EP 0 509 328 A2 discloses an arrangement of 3 SAWs, which are arranged, however, in series. This results in different flow conditions in the individual sensors.
EP 0 477 684 A1 discloses an arrangement of more than 2 SAWs with different coating. The sensors are not arranged however in any particular way.
If several oscillators are operated in a small space at almost the same frequency, there is a cross-influence as a result of the electromagnetic transgressions. In an extreme case, this may result in a lock-in situation (oscillators vibrate at the same frequency). This behavior limits the possibilities of miniaturization of HF oscillators with similar frequencies. The problem can be circumvented if the oscillators are switched on and off. When switched on again, the oscillators drift because of thermal changes whereby the measuring sensitivity of for example a sensor is reduced. In order to avoid this, it would be necessary to wait after each switching on until a thermal equilibrium has been established. This however results in a long down time.
It is the object of the present invention to provide a sensor of the type described above, wherein, however, electromagnetic transgression of the oscillators is avoided without sensitivity losses and extended down times.